1. Field of the Invention
The present invention relates generally to a hydraulic pressure control valve and more specifically to a pressure control valve for an "X" type dual circuit hydraulic braking system which valve includes a fail-safe feature in the event of a failure in one circuit.
2. Description of the Prior Art
In a previously proposed arrangement each circuit of an "X" type dual circuit arrangement has been provided with a valve unit of the type shown in FIG. 1 of the drawings. Each of these valve units includes a proportioning valve which modulates the pressure transmitted therethrough subsequent to a modulation point being reached, and a so called "G" valve which functions to sense the degree of deceleration of the vehicle and, upon a given deceleration of the vehicle being sensed, induces the proportioning valve to commence modulating the pressure transmitted therethrough. Viz., upon applying the brakes of a vehicle sufficiently to induce the aforementioned given degree of deceleration, a "G"-ball 1 rolls up a predetermined inclination to contact and close a valve seat 2 thus isolating an actuator piston 3 from a source of pressure P1 (from a tandem master cylinder). This traps a predetermined hydraulic pressure PG in the chamber to which the actuator piston 3 is exposed. Upon this trapping, the actuator piston ceases to stroke against the bias of springs 4 and 5 whereby a constant load is applied to a plunger 6 of the proportioning valve through said springs and the split or modulation point of the proportioning valve is determined.
Under the above conditions as the hydraulic pressure P1 rises, the plunger 6 is moved leftwardly against the combined bias of the springs 4 and hydraulic pressure PG acting on the actuator piston 3 by the pressure in the chamber 7 on the right-hand side of the land 8 formed on the plunger 6. This moves the stem of a poppet valve 9 out of contact with the end plug 10 of the proportioning valve bore allowing the poppet valve 9 of close and temporarily inhibit the increase in the pressure P2.
Following the closure of the poppet valve the pressure P1 increases in the chamber 11 defined on the left-hand side of the land 8 which moves the plunger back against the pressure trapped in the chamber 7, thus bringing the poppet valve stem back into contact with the end plug 10 re-opening the poppet valve. The plunger 6 is thus reciprocated rapidly back and forth, repeatedly opening and closing of the poppet valve and accordingly modulating the pressure P.sub.2 transmitted through the valve to a rear wheel brake cylinder in a desired skid preventing manner.
Now, while this arrangement has provided desirable braking characteristics under normal conditions, it has suffered from the drawbacks that two relatively expensive and heavy valves must be incorporated into the system and, more importantly, in the event of the failure of one of the two circuits, the valve disposed in the operative circuit still functions normally inducing a pressure reduction to the rear wheel brake cylinder with which it is associated. Accordingly, even when it is desired to apply the maximum braking pressure to the now single operative rear wheel brake cylinder, the modulation of the pressure to said cylinder still occurs reducing the braking capacity under such conditions.